1. Field of the Invention
The present invention relates to an oxadiazole derivative, and a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the oxadiazole derivative.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In the basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Since such light-emitting elements are self-luminous elements, it has advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, light-emitting elements are thought to be suitable for flat panel display elements. Light-emitting elements are also highly advantageous in that they can be thin and lightweight. Furthermore, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission; thus, large-area elements utilizing planar light emission can be easily formed. This is a difficult feature to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, these carriers (i.e., electrons and holes) are recombined, whereby the light-emitting organic compound is excited. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light. Note that the excited state of an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. The statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound that is capable of converting energy of a singlet excited state into luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Thus, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% on the basis of S*:T*=1:3.
In contrast, with a compound that can convert energy of a triplet excited state into luminescence (hereinafter, called a phosphorescent compound), the internal quantum efficiency can be increased to 75% to 100% in theory. In other words, an element using a phosphorescent compound can have three to four times as high emission efficiency as that of an element using a fluorescent compound. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly-efficient light-emitting element (e.g., see Non-Patent Document 1).
When formed using the above-described phosphorescent compound, a light-emitting layer of a light-emitting element is often formed such that a phosphorescent compound is dispersed in a matrix of another compound in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound. Here, the substance serving as a matrix is called a host material, and the substance dispersed in a matrix, such as a phosphorescent compound, is called a guest material.
A host material needs to have higher triplet excitation energy (an energy difference between a ground state and a triplet excited state) than a phosphorescent compound in the case where the phosphorescent compound is a guest material. In addition, the host material needs to have a carrier transport property by which desired carrier balance can be controlled in a light-emitting layer. With the use of such a host material, characteristics of a light-emitting element can be improved.